1. Field of the Invention
The present invention is directed to an improved vinyl polymer composition and the method of making same. More particularly, the present invention is directed to an improved vinyl polymer composition which incorporates an ethylene-vinyl acetate copolymer and process for making same.
2. Background of the Prior Art
Vinyl polymers, of which polyvinyl chloride is the most common, are commonly employed in a myriad of applications. No small reason for this wide acceptance is the incorporation of additives that specially modify the polymer to satisfy various property requirements. For example, vinyl polymers, especially polyvinyl chloride (PVC), are sensitive to heat. Therefore, a heat stabilizer is usually added to PVC to form a heat stable PVC composition. Lubricants form an essential component in many vinyl polymer compositions in that these compositions are oftentimes extruded or injected molded. External lubricants serve to reduce the polymer's tendency to stick to hot metal surfaces while internal lubricants increase the flow of the vinyl polymer particles within the melt. Pigments and colorants are added to provide the desired color to the polymer. An important attribute of rigid vinyl polymers is toughness. Toughness is increased with the addition of an impact modifier. Other additives may optionally also be incorporated in vinyl polymer compositions. For example, fillers, which reduce the cost and decrease gloss, are, in many cases, incorporated in these compositions.
The blending of these additives into vinyl polymers to produce a uniform composition represents a major, technical undertaking. That is, with the incorporation of these various additives, the mixing together of the components into a melt at sufficiently low viscosity represents a difficult technological development. To assist in decreasing the melt viscosity to a level which is processible by various mixing apparatus, is the function of processing aids. These processing aids are added to the composition to substantially improve the processing behavior of the vinyl composition. Processing aids provide this function by decreasing melt viscosity. Among the processing aids commonly added to vinyl polymer compositions to decrease processing difficulties are styrene-acrylonitrile copolymers and acrylic polymers.
Although these processing aids permit blending of the components into a vinyl polymer composition they do not function as efficiently as could be desired. This lack of processing efficiency detracts from certain critical properties in the rigid vinyl polymer product. These adverse effects are caused by the inability of the processing aids of the prior art to effect rapid fusion at low melt viscosity. As a result, compositions employing these processing aids of the prior art require higher blending temperatures than would otherwise be the case. This processing limitation adversely affects the physical properties of the resultant vinyl composition. Vinyl polymer compositions formulated with processing aids of the prior art are thus oftentimes characterized by inadequate and non-uniform impact strengths. In addition, surface appearance of these prior art rigid vinyl polymer compositions leave much to be desired.
The use of ethylene-vinyl acetate copolymers as an additive in vinyl polymer compositions is known in the art. However, the ethylene-vinyl acetate copolymers provided in the prior art vinyl polymer compositions were employed to improve impact strength. For example, U.S. Pat. No. 3,125,545 discloses an improved impact strength vinyl chloride resin using about 2 to 50 percent by weight of an ethylene-vinyl alkanoate copolymer as an impact improver.
U.S. Pat. No. 3,283,035, also representative of the prior art, describes a polyvinyl chloride resin provided with increased impact strength by the addition of an ethylene-vinyl alkanoate copolymer and a chlorinated polyolefin.
U.S. Pat. No. 3,457,330 teaches a resin composition having improved impact strength which is a ternary mixture of a vinyl chloride resin, an ethylene-vinyl acetate copolymer and polyethylene.
U.S. Pat. No. 3,517,083 sets forth a vinyl chloride polymer having improved impact strength which comprises a blend of 1 to 50 parts by weight of a solid ethylene-vinyl acetate copolymer and 50 to 99 parts by weight of a vinyl chloride polymer.